Type: propositional reasoning in XPath
Description: queries are either tautologies or contradictions. A static
analysis of the query might significantly reduce the amount of dynamic
evaluation.
Target documents: any XMark document
Queries: 
TAUTO) The filter is a tautology (always true) hence all open auctions
must be returned:

/site/open_auctions/open_auction[
(not(bidder/following::bidder) or not(bidder/preceding::bidder)) or
(bidder/following::bidder and bidder/preceding::bidder)]/interval

CONTRA) The filter is a contradiction (always false) hence no open
auction must be returned

/site/open_auctions/open_auction[
(not(bidder/following::bidder) or not(bidder/preceding::bidder)) and
(bidder/following::bidder and bidder/preceding::bidder)]/interval